Hormones control cellular replication in target tissues as well as differentiated function. The effects of hormones on mitosis are particularly important with respect to hormone-dependent malignancies. We will study the mechanisms whereby hormones mediate mitosis in breast tumor tissue and utilize these properties to manipulate cell replication. In hormone-responsive human breast tumors, estradiol predominates as the hormone which stimulates cellular growth. In this proposal, we plan to investigate its direct effect in vitro on breast cancer mitogenesis. In addition, we will study the role of polyamines as mediators of estrogen action. We plan to conduct our experiments in the N-nitrosomethylurea (NMU)-induced rat mammary tumor. This experimental model resembles human breast cancer and can be successfully grown in a soft agar clonogenic assay. This in vitro method offers the advantage over standard tissue culture techniques of a direct tool for the study of tumor cell heterogeneity. We will then utilize the mitogenic effects of estrogens and polyamines to manipulate tumor growth in the attempt to enhance sensitivity to cytotoxic chemotherapy. We believe that a major obstacle to the effects of chemotherapy is tumor cell heterogeneity. Cytotoxic drugs, in fact, exert a tumoricidal effect preferentially on rapidly dividing cells. At any point in time, however, these cells represent a small fraction of the overall malignant cell population. We will synchronize and recruit tumor cells by sequential administration of antiestrogen and/or polyamine inhibitors, followed by estrogens and/or polyamines in the attempt to increase the pool of rapidly dividing cells sensitive to chemotherapy. Success in achieving cell synchronization will be documented by autoradiographic techniques. Subsequent experiments in vivo will establish whether responsiveness to chemotherapy is enhanced by previous synchronization and recruitment of tumor growth. This proposal will deal with basic aspects of breast cancer mitogenesis applied to the development of new and improved treatment strategies.